Unable to install add-ons (extensions or themes)

An inability to install an extension can be the result of problems with the extension, Firefox preferences, or the version of Firefox you are using. This article describes various causes of install problems and what you can do to address them.

Add-on is not compatible

An internal file in each add-on indicates the range of Firefox versions with which the extension is compatible. In the Firefox Add-ons area of Mozilla, you can review version compatibility information on the page for each available extension.

If your installed version of Firefox does not fit within that range, installing the add-on results in an error, indicating that the add-on is not compatible. If your version of Firefox is incompatible with the add-on, you can:

Add-on is blocked

Firefox prevented the site from asking you to install software

By default, Firefox permits you to install add-ons only from trusted websites. If the website from which you are installing add-on is not on the trusted websites list, a yellow bar appears across the top of the web page with a message similar to the following:

Firefox prevented this site (www.example.com) from asking you to install software on your computer.

To allow the site, click Allow in the yellow bar.

To add the site to the trusted sites list:

At the top of the Firefox window, click on the Firefox button and then select Options.At the top of the Firefox window, click on the Tools menu and then select Options.On the menu bar, click on the Firefox menu and select Preferences....At the top of the Firefox window, click on the Edit menu and select Preferences.

Click the menu button
and choose Options.Preferences.

Select the Security panel.

At the right of Warn me when sites try to install add-ons, click Exceptions….

In the Allowed Sites - Add-ons Installation dialog, the list of permitted sites is displayed. The current site URL is inserted into the text field.

To allow the site, click Allow. Then, click Close.

Click OK to close the Options window.Click Close to close the Preferences window.Close the Preferences window.

You are asked to download the add-on, rather than installing it

Some websites serve add-ons as downloads, requiring you to download the file to your computer, before installing it.

At the top of the Firefox window, click on the Firefox buttonOn the menu bar, click on the Tools menuAt the top of the Firefox window, click on the Tools menu, and then click Add-ons. The Add-ons Manager tab will open.Click the menu button
and choose Add-ons. The Add-ons Manager tab will open.

In the Add-ons Manager tab, select the Extensions panel.

To add the downloaded add-on to the list of available add-ons, drag and drop the file into the Add-ons window. The add-on is added to the list.

The installation process should begin.

Download Error -228

Disabled cache

This error message may be caused when the Firefox cache is disabled. To re-enable the Firefox cache:

At the top of the Firefox window, click on the Firefox button and then select Options.At the top of the Firefox window, click on the Tools menu and then select Options.On the menu bar, click on the Firefox menu and select Preferences....At the top of the Firefox window, click on the Edit menu and select Preferences.

Click the menu button
and choose Options.Preferences.

Select the Advanced icon.

Click on the Network tab.

In the Cache section, the size of the cache must be at least 1 MB.

Click OK to close the Options window.Click Close to close the Preferences window.Close the Preferences window.

Other software

Sometimes, firewalls or spyware screeners may cause the Download Error -228 message. If you use the CA firewall, Windows Defender, or the resident option of Spybot S&D ("Teatimer" feature), you should temporarily disable the software to see if your system permits the add-on to install. For more information, see the documentation that came with your software.

Nothing happens when you click the install link

Corrupt extension files

Corrupt extension configuration files can cause problems when trying to install an extension. Deleting these files will make Firefox recreate them (removing any corruption) the next time it starts. You won't lose your extensions or their settings by deleting these files.

Open your profile folder:

At the top of the Firefox window, click on the Firefox button, go over to the Help menuOn the menu bar, click on the Help menuAt the top of the Firefox window, click on the Help menu and select Troubleshooting Information. The Troubleshooting Information tab will open.Click the menu button
, click help
and select Troubleshooting Information. The Troubleshooting Information tab will open.

Under the Application Basics section, click on Show FolderShow in FinderOpen Directory. A window with your profile filesfolder will open.

At the top of the Firefox window, click on the Firefox button and then select ExitAt the top of the Firefox window, click on the File menu and then select ExitOn the menu bar, click on the Firefox menu and select Quit FirefoxAt the top of the Firefox window, click on the File menu and select Quit

Click the menu button
and then click ExitQuit

Delete the following files:

extensions.sqlite

extensions.sqlite-journal (if found)

extensions.ini

Restart Firefox.

Open your profile folder:

At the top of the Firefox window, click on the Firefox button, go over to the Help menuOn the menu bar, click on the Help menuAt the top of the Firefox window, click on the Help menu and select Troubleshooting Information. The Troubleshooting Information tab will open.Click the menu button
, click help
and select Troubleshooting Information. The Troubleshooting Information tab will open.

Under the Application Basics section, click on Show FolderShow in FinderOpen Directory. A window with your profile filesfolder will open.

At the top of the Firefox window, click on the Firefox button and then select ExitAt the top of the Firefox window, click on the File menu and then select ExitOn the menu bar, click on the Firefox menu and select Quit FirefoxAt the top of the Firefox window, click on the File menu and select Quit

Click the menu button
and then click ExitQuit

Delete the following files:

extensions.sqlite

extensions.sqlite-journal (if found)

extensions.ini

extensions.json

Restart Firefox.

Conflict with other extensions

Another installed extension may interfere with the installation of new extensions. Install the new extension in Firefox Safe Mode as a workaround.